Continuously variable transmissions are stepless, adjustable, automatic transmissions. The stepless adjustment, i.e. shifting, is carried out by the axial displacement of two slidable, V-pulley disks of the variator acting on respective primary and secondary shafts. In this way, a change in the relationship between the highest possible ratio (i.e. low gear) and the smallest possible ratio (direct drive or overdrive) can be brought about.
In order to achieve the necessary frictional force on the torque converter, which in general is a steel thrust belt to transfer the motor torque, a continuing, specified, compression on the V-pulley disks is necessary. The shift of the V-pulley disks occurs by means of the application of an additional displacing pressure-increment on one of the two slidable V-pulley disks.
In the case of modes of construction of the CVT, known in practice, a pressure cylinder is placed on each adjustable V-pulley disk as a source of pressure apparatus, which undertakes both the necessary continuous application of pulley-pressure as well as shifting the V-pulley disk for the ratio change.
In this way, a completely independent control pressure is applied to each of the two V-pulley disks, and a necessary flow for shifting the variator is made available to the slidable V-pulley disks, likewise independent of one another.
Thus, for example, in a shift from the position LOW to the position OD with a higher gear, a first V-pulley disk is loaded with an oil volume and its holding-chamber is filled. Simultaneously, a second flow out of a second V-pulley disk of the other set of V-pulley disks is diverted into an oil receiver.
Analogous to this, by shifting the variator from an OD condition in the direction of the LOW ratio, the second V-pulley disk is filled causing the flow from the first V-pulley disk to be returned to the oil tank.
Since, in the case of stepless transmissions, except in a rare occurring constant driving situation, a ceaseless change of gear ratio is in progress, this means that continually, oil in the low pressure side must be deprived of pressure and is lost into the oil tank while, at the same time, oil with the necessary high pressure on the high pressure side must be transported.
This has the disadvantage that the flow relationships resulting from this, are geometrically strongly non-linear and necessary pump work must be carried out for the assurance of a high shifting dynamic by correspondingly large dimensioned pumps. The overall efficiency is thereby negatively affected, and the driving capacity of the motor is diminished by the power consumption of the pumps.
DE 41 31 931 discloses a stepless transmission, in which case one contact pressure cylinder each supplies the said necessary continuous pulley-pressure on the two displaceable V-pulley disks of the variator. For the sliding of the pulleys, another displacement cylinder is provided separately therefrom. In this arrangement, the cylinder chambers of the two contact-pressure cylinders are connected by a pressure compensation piping.
Since the degree of the contact-pressure for the assurance of the transmission of torque at the variator is dominant in relation to the pressure difference for shifting, which is necessary for the change of ratio, i.e. gears, the contact-pressure is generated by large effective piston areas in both of the inner cylinder chambers and so the pressure difference for shifting with small effective piston areas can be realized. In the shifting cylinders, the volumes exchange themselves upon a variator shift and the oil is lost into an oil tank. In the contact-pressure cylinders, the volumes do not vary, and the oil is only exchanged between the opposed chambers, whereby said oil remains in the system and does not have to be retransported.
This known solution, by means of the mechanics of the double V-pulley disks with outside cylinders and interconnected inner cylinders, enables a reduction of the hydraulic losses in the case of a variator displacement. However, the construction of this solution is complicated and expensive.
Thus, the purpose of the present invention is to create a continuously variable transmission with a variator of simple construction in which the hydraulic capacity losses are minimized by the control of said variator.
In accordance with the invention, this purpose is achieved by the features named in Claim 1.